1. Field of the Invention
The present invention relates to a toner supply device and an image forming apparatus incorporating such a toner supply device.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a facsimile, a printer or a copying machine, the amount of toner consumed in a development device is supplied from a toner storage portion, and thus the concentration of toner within the development device is maintained. The toner may be supplied from the toner storage portion to the development device by the rotation of a transport screw.
It is important to stabilize the amount of toner transported by the transport screw not only to maintain the concentration of toner within the development device but also to estimate the amount of toner left in the toner storage portion from a period of time during which the transport screw is driven.
Hence, for example, document 1 (Japanese Unexamined Patent Application Publication No. 2006-126433) proposes a technology in which a cylindrical portion including a transport screw is provided substantially in a horizontal direction such that its length is one to three times as great as the pitch of the transport screw. Moreover, document 2 (Japanese Unexamined Patent Application Publication No. 2008-287214) discloses a technology in which a toner transport path is inclined such that a downstream side in a toner transport direction is higher than an upstream side, and in which, while a transport screw is stopped, toner is prevented from dropping to a development device due to its weight. Furthermore, document 3 (Japanese Unexamined Patent Application Publication No. 2009-210721) discloses a technology in which a force that compresses toner by a transport unit is increased as a toner transport direction extends to a downstream side, and also discloses, as one method of increasing the toner compressing force, a configuration in which the pitch of the blades of a transport screw is decreased as the toner transport direction extends from the upstream side to the downstream side.
However, even with the technologies disclosed in patent documents 1 and 2, the amount of transport of the toner with the transport screw is not sufficiently stabilized. In the technology disclosed in patent document 3, although, when the amount of toner in the toner storage portion is decreased, the amount of transport of the toner is probably stabilized, when a large amount of toner is stored in the toner storage portion, it is likely that, since excessive toner taken in an entrance portion of the transport screw cannot be returned to the toner storage portion, the amount of transport of the toner is varied.
In view of the foregoing conventional problems, the present invention is made; an object of the present invention is to stabilize the amount of transport of toner with a transport screw, both in a toner supply device that supplies, using the transport screw, toner from a toner storage portion to a development device and in an image forming apparatus.